Imaging method and device using shearing waves

ABSTRACT

The invention concerns an image method for observing the propagation of low-frequency shearing pulse wave simultaneously in multiple points of a diffusing viscoelastic medium. The method consists in transmitting at a very high rate ultrasonic compression waves enabling to obtain a succession of images of the medium; then in delayed processing of the resulting images by intercorrelation to determine in each point of each image the movements of the medium while the shearing wave is being propagated.

FIELD OF TITLE INVENTION

The present invention relates to imaging methods and devices using shearwaves.

More particularly, the invention relates to a method of imaging usingshear waves to observe a diffusing viscoelastic medium which containsparticles reflecting ultrasonic compression waves, in which method anelastic shear wave is generated in the viscoelastic medium and thedisplacement of the viscoelastic medium subjected to said shear wave isobserved by means of at least one ultrasonic compression wave.

BACKGROUND OF THE INVENTION

Document U.S. Pat. No. 5,810,731 describes an example of such a method,in which the shear wave is generated locally inside the observedviscoelastic medium, by means of the radiation pressure of a modulatedultrasound wave focussed on a point to be observed. An additionalultrasound wave is then dispatched to this focal point, the reflectionof the wave making it possible to ascertain certain propagationparameters of the shear wave (in particular the dynamic viscosity of themedium and its shear modulus) in the vicinity of the abovementionedfocal point.

This technique has the drawback of allowing the analysis of just asingle point of the viscoelastic medium under study each time a shearwave is generated. If one wishes to obtain a complete image of theobserved viscoelastic medium, it is necessary to repeat the operation avery large number of times, this involving a considerable idle time (forexample, several minutes) to obtain this image.

This considerable idle time renders this prior art method impractical touse.

Moreover, such an idle time may impede the use of said method to obtainan image of a living tissue, which is always in motion.

OBJECT AND SUMMARY OF THE INVENTION

An aim of the present invention is in particular to alleviate thesedrawbacks.

To this end, according to the invention, a method of the kind inquestion is essentially characterized in that the shear wave isgenerated by applying to the viscoelastic medium an excitation havingthe form of a low-frequency pulse which exhibits a central frequency fof between 20 and 5000 Hz, this low-frequency pulse exhibiting aduration of between 1/2f and 20/f, in that it comprises a propagationobservation step in the course of which the propagation of the shearwave is observed simultaneously at a multitude of points in the observedmedium, these points forming a substantially continuous observationfield extending at least along a first axis, this shear wave propagationobservation step consisting in:

emitting into the observed medium a succession of at least 10 shots ofultrasonic compression waves at a rate of between 100 and 100 000 shotsper second,

detecting and recording in real time the echoes generated by thereflecting particles of the viscoelastic medium at each ultrasonic waveshot, these echoes corresponding (directly or indirectly) to successiveimages of the observed medium, and in that said method furthermorecomprises a subsequent image processing step in the course of which theimages thus obtained are processed at a later time at least bycross-correlation between successive images, so as to determine at eachpoint of the observation field a motion parameter chosen between thedisplacement and the strain of the viscoelastic medium, in such a way asto thus obtain a succession of images showing the evolution of themotion parameter of the viscoelastic medium under the effect of thepropagation of the shear wave.

By virtue of these arrangements, a film is obtained which clearlyillustrates the propagation of the shear wave in the viscoelasticmedium, which may make it possible for example, in medical applications,to directly tag cancerous areas in the tissues of a patient: thepropagation of the shear waves in fact occurs there very differentlyfrom the neighboring areas.

This tagging is performed much more easily than through conventionalobservation by simple ultrasound echography, since the propagation ofthe shear waves is dependent on the shear modulus of the medium, itselfvarying greatly between an area of healthy tissues and an area ofcancerous tissues: the shear modulus typically varies in a ratio of 1 to30 between a healthy area and a cancerous area, whereas the bulkmodulus, which governs the propagation of the acoustic compression wavesused in ultrasound echography, varies by only the order of 5% between ahealthy tissue and a cancerous tissue.

It will be noted that the film obtained illustrates the propagation ofthe shear wave much more clearly than the simple succession of imagesgiven by the reflecting particles of the medium, since said film makesit possible to view at each instant the areas of the observed mediumwhich undergo motions of the same magnitude on account of thepropagation of the shear wave, whereas the succession of images of thereflecting particles would make it possible to view only a haze ofbright points in motion.

In preferred embodiments of the method according to the invention,recourse may furthermore possibly be had to one and/or to the other ofthe following arrangements:

the duration of the low-frequency pulse is between 1/2f and 2f;

the central frequency of the low-frequency pulse is between 30 and 1000Hz;

the observed viscoelastic medium consists of a living body comprising atleast one internal organ subjected to pulsatile motions, thelow-frequency pulse which generates the shear wave being constituted bya pulsatile motion of said internal organ;

the observed viscoelastic medium is delimited by an outside surface andthe low-frequency pulse is applied in the vicinity of this outsidesurface;

the low-frequency pulse is applied by a means of excitation chosen from:

an acoustic wave generated by at least one acoustic transducer,

and a shock generated locally by physical contact in the vicinity of theoutside surface of the viscoelastic medium;

the ultrasonic compression wave shots are emitted and the echoesgenerated by the reflecting particles of the viscoelastic medium aredetected by means of a bank of transducers which comprises at least onetransducer and which is arranged in contact with the outside surface ofthe viscoelastic medium, the shear wave being applied to theviscoelastic medium by imposing a pulsatile displacement on said bank oftransducers;

said motion parameter is the strain of the viscoelastic medium: thisarrangement is particularly useful in the last case envisagedhereinabove, since it makes it possible to dispense with thedisplacement of the bank of transducers, which displacement wouldotherwise disturb the measurement of the displacement of the points ofthe observation field;

in the course of the observation of the propagation of the shear wave,between 100 and 10 000 ultrasonic compression wave shots are emitted ata rate of between 100 and 100 000 shots per second;

the observation field extends at least along a plane comprising on theone hand, the first axis and on the other hand, a second axisperpendicular to the first axis;

in the course of the propagation observation step, a bank of severalacoustic transducers arranged at least along the second axis is used toemit the ultrasonic compression wave shots and detect the echoesgenerated by the reflecting particles of the viscoelastic medium, theechoes detected by each acoustic transducer being stored directlywithout prior processing in the course of the propagation observationstep, and the image processing step comprising a preliminary substep offorming paths in the course of which an image of the viscoelastic mediumcorresponding to each ultrasonic compression wave shot is generated bycombining at least some of the echoes received by the varioustransducers;

the image processing step is followed (immediately or otherwise) by aviewing step in the course of which a film consisting of the successionof processed images is viewed under slow motion, each point of eachimage exhibiting an optical parameter which varies according to thevalue of the motion parameter assigned to this point;

the optical parameter is chosen from the gray level and the chromaticlevel;

the image processing step is followed (immediately or otherwise) by amapping step in the course of which, on the basis of the evolution ofthe motion parameter over the course of time in the observation field,at least one propagation parameter is calculated for the shear wave atat least some points of the observation field;

the shear wave propagation parameter which is calculated in the courseof the mapping step is chosen from the speed of the shear waves, theshear modulus, the attenuation of the shear waves, the shear elasticity,and the shear viscosity. Moreover, a subject of the invention is also animaging device using shear waves to observe a diffusing viscoelasticmedium which contains particles reflecting ultrasonic compression waves,this device comprising means of excitation for generating an elasticshear wave in the viscoelastic medium and means of acquisition forobserving, by means of at least one ultrasonic compression wave, thedisplacement of the viscoelastic medium subjected to said shear wave,characterized in that the means of excitation are adapted to apply tothe viscoelastic medium an excitation having the form of a low-frequencypulse which exhibits a central frequency f of between 20 and 5000 Hz,this low-frequency pulse exhibiting a duration of between 1/2f and 20/f,and in that the means of acquisition are adapted to observe thepropagation of the shear wave simultaneously at a multitude of points inthe observed medium, these points forming a substantially continuousobservation field extending at least along a first axis, said means ofacquisition being adapted for:

emitting into the observed medium a succession of at least 10 shots ofultrasonic compression waves at a rate of between 100 and 100 000 shotsper second,

detecting and recording in real time the echoes generated by thereflecting particles of the viscoelastic medium at each ultrasonic waveshot, these echoes corresponding (directly or indirectly) to successiveimages of the observed medium, and in that said device furthermorecomprises image processing means adapted to process at a later time theimages obtained by the observation means, at least by cross-correlationbetween successive images, so as to determine at each point of theobservation field a motion parameter chosen between the displacement andthe strain of the viscoelastic medium, in such a way as to thus obtain asuccession of images showing the evolution of the motion parameter ofthe viscoelastic medium under the effect of the propagation of the shearwave.

In preferred embodiments of the device according to the invention,recourse may moreover possibly be had to one and/or to the other of thefollowing arrangements:

the means of observation comprise a bank of transducers which includesat least one transducer and which is adapted to be arranged in contactwith an outside surface delimiting the viscoelastic medium, the means ofexcitation being adapted to impose a pulsatile displacement on said bankof transducers;

said motion parameter is the strain of the viscoelastic medium;

the observation field extends at least along a plane comprising on theone hand, the first axis and on the other hand, a second axisperpendicular to the first axis, the bank of transducers comprisingseveral transducers arranged at least along the second axis, controlmeans being provided for selectively operating the device either in themode of imaging by shear waves, or in a standard echography mode makingit possible to acquire between 10 and 100 images per second.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent in the course of the following description of several of itsembodiments, given by way of nonlimiting examples, in conjunction withthe appended drawings.

In the drawings:

FIG. 1 is a schematic view of a device for imaging by shear wavesaccording to one embodiment of the invention,

FIG. 2 is a detail view showing a variant of the device of FIG. 1.

MORE DETAILED DESCRIPTION

In the various figures, the same references designate identical orsimilar elements.

FIG. 1 represents an example of a device for imaging by shear wavesaccording to the invention, for studying the propagation of elasticshear waves in a viscoelastic medium 1 which is diffusing with regard toultrasonic compression waves, and which may for example be:

an inert body, in particular in the case of quality control forindustrial applications, in particular agro-food applications,

or a living body, for example a part of the body of a patient, in thecase of medical applications.

This device comprises an acoustic transducer such as a loudspeaker 2 ora vibrator can which is arranged against the outside surface 3 of theobserved medium 1, this surface 3 consisting for example of the skin ofthe patient in medical applications.

The loudspeaker 2 can be controlled by a microcomputer 4, for example byway of a low-frequency pulse generator circuit G (this circuit canconsist in particular of the sound card of the microcomputer 4) and ofan amplifier A, so as to apply an excitation in the form of alow-frequency pulse to the surface 3 of the observed medium, so as togenerate a shear wave in the viscoelastic medium 1.

This low-frequency pulse generally exhibits an amplitude which may be ofthe order of 1 mm and a central frequency f of between 20 and 5000 Hz,applied for a duration of between 1/2f and 20/f. Preferably, theduration of application of the low-frequency pulse is between 1/2f and2f and the frequency f is between 30 and 1000 Hz, this frequencytypically being of the order of 50 Hz.

As a variant, the acoustic shear wave could also be obtained (with theabovementioned amplitude and frequency characteristics):

via a shock generated locally by physical contact in the vicinity of theoutside surface of the viscoelastic medium, via at least one automaticmechanical actuator controlled by the microcomputer 4,

via a shock applied manually by physical contact in the vicinity of theoutside surface of the viscoelastic medium,

or else, via a natural pulsatile motion of an internal organ of thehuman or animal body (for example a beating of the heart) in medicalapplications.

The elastic shear wave produced by the loudspeaker 2 moves with arelatively low speed Cs, of the order of a few m/s (typically, from 1 to10 m/s in the human body), producing internal motions in the observedviscoelastic medium 1.

These motions are followed by dispatching into the medium 1, ultrasoniccompression waves which interact with the diffusing particles 5contained in the medium 1, which particles are reflecting in respect ofultrasonic compression waves. The particles 5 may be constituted by anyheterogeneity of the medium 1, and in particular, when dealing with amedical application, by collagen particles present in human tissues.

To observe the propagation of the shear wave, use is therefore made ofan ultrasound probe 6 arranged against the outside surface 3 of theobserved medium 1. This probe dispatches, along an axis X, pulses ofultrasonic compression waves of the type of those commonly used inechography, at a frequency of for example between 1 and 100 MHz andpreferably between 3 and 15 MHz. It will be noted that the probe 6 canbe arranged:

either on the same side of the medium 1 as the loudspeaker 2, asrepresented in FIG. 1,

or on the opposite side from the loudspeaker 2 with respect to themedium 1,

or in any other position, for example in a transverse arrangement withrespect to the loudspeaker 2.

The ultrasound probe 6 consists of a bank of n ultrasound transducersT1, T2, . . . , Ti, . . . , Tn, n being an integer number n at leastequal to 1.

This probe 6 usually takes the form of a linear strip which can comprisefor example n=128 transducers aligned along an axis Y perpendicular tothe axis X, simultaneously dispatching their ultrasound wave pulses insuch a way as to generate a “plane” wave (that is to say in thisinstance a wave whose wave front is straight in the X, Y plane) or anyother type of wave illuminating the entire observation field.

As a variant, the bank 2 of transducers may possibly be reduced to asingle transducer T1, or conversely take the form of a two-dimensionalarray extending for example along a plane perpendicular to the X axis.

Each of the transducers T1, T2, . . . Tn is controlled by themicrocomputer 4 or by a central processing unit CPU (which is containedfor example in an electronic rack 7 linked by a flexible cable to theprobe 6), so as to emit successive shots of ultrasonic compression wavesinto the medium 2, in the course of an observation phase which may lastfor example less than one second and in the course of which p shots ofultrasonic compression waves are emitted (p being an integer lyingbetween 100 and 10 000 and preferably between 1000 and 100 000), at arate of between 100 and 100 000 shots per second and preferably between1000 and 100 000 shots per second, in particular between 1000 and 10 000shots per second (this rate is limited by the time for theoutward/return journey of the compression wave in the medium 1, hence bythe thickness of the medium 1 in the direction X: it is in factnecessary for all the echoes generated by the compression wave to havebeen received by the probe 6 before a new compression wave isdispatched).

The ultrasonic compression wave shots of the observation phasepreferably begin just before the emission of the shear wave.

Moreover, in the case where the shear wave is generated by a pulsatilemotion of an organ of a living body, it is advantageously possible tosynchronize the starting of the ultrasonic compression wave shots withthis pulsatile motion. For example in the case where the shear wave isgenerated by a heartbeat, it is possible to synchronize the starting ofthe ultrasonic compression wave shots with a chosen phase of theelectrocardiogram.

Each of these shots gives rise to the propagation of an ultrasoniccompression wave in the medium 1, with a much higher speed ofpropagation than the shear waves, for example of the order of 1500 m/sin the human body.

The ultrasonic wave thus generated interacts with the reflectingparticles 5, thereby generating echoes or other similar disturbances ofthe signal, which are known per se as “speckle noise” within the fieldof echography.

This “speckle noise” is picked up by the transducers T1, . . . , Tnafter each shot. The signal sij(t) thus picked up by each transducer Tiafter shot No. j is firstly sampled at high frequency (for example from30 to 100 MHz) and digitized in real time (for example on 8 bits, or incertain cases on 1 bit) by a sampler belonging to the rack 7 and linkedto this transducer, respectively E1, E2, . . . En.

The signal sij (t) thus sampled and digitized is then stored, also inreal time, in a memory Mi belonging to the rack 7 and specific to thetransducer Ti.

Each memory Mi exhibits for example a capacity of the order of 1 Mb, andcontains all the signals sij (t) received successively for shots j=1 top.

At a later time, after the storage of all the signals sij(t)corresponding to one and the same shear wave propagation, the centralprocessing unit CPU has these signals reprocessed by a summator circuitS belonging to the rack 7 (or else it performs this processing itself,or else said processing can be performed in the microcomputer 4),through a conventional process of path formation.

Signals Sj (x, y) are thus generated, each corresponding to the image ofthe observation field after shot No. j.

For example, a signal Sj (t) can be determined through the followingformula:${{Sj}\quad (t)} = {\sum\limits_{i = 1}^{n}\quad {\alpha_{i}\quad {\left( {x,y} \right) \cdot {{sij}\left\lbrack {{t\quad \left( {x,y} \right)} + {d_{i}\quad {\left( {x,y} \right)/V}}} \right\rbrack}}}}$

where:

sij is the raw signal sensed by transducer No. i after ultrasoniccompression wave shot No. j,

t(x,y) is the time taken by the ultrasonic compression wave to reach thepoint of the observation field with coordinates (x,y), with t=0 at thestart of shot No. j,

di(x,y) is the distance between the point of the observation field withcoordinates (x,y) and transducer No. i, or an approximation of thisdistance,

V is the average speed of propagation of the ultrasonic acousticcompression waves in the observed viscoelastic medium,

and αi(x,y) is a weighting coefficient allowing for apodization laws (inpractice, in many cases it will be possible to consider that αi(x,y)=1).

The above formula applies mutatis mutandis when the observation field is3-dimensional (plane array of transducers), on replacing the spatialcoordinates (x,y) by (x,y,z)

When the probe 6 comprises just one transducer, the path formation stepis unnecessary, and we have directly Sj (x)=sj [2.x/V], with the samenotation as above.

After the path formation step, should there be one, the centralprocessing unit CPU stores in a central memory M belonging to the rack 7the image signals Sj (x,y) or Sj(x) or Sj(x,y,z), which each correspondto shot No. j. These signals may also be stored in the microcomputer 4when it performs the image processing itself.

These images are then processed in pairs, again at a later time, bycross-correlation. This cross-correlation can be carried out in a DSPcircuit belonging to the rack 7, or be programmed into the centralprocessing unit CPU or into the microcomputer 4.

By way of example, this cross-correlation can be done by comparing thesignals Sj (x,y) and Sj+1 (x,y) (in the case of a two-dimensionalobservation field) over sliding spatial windows of predetermined lengthΔx, which may range for example from λ to 10λ, where λ is the wavelengthof the ultrasonic compression waves (i.e. around 0.42 to 4.2 mm at 3.5MHz in water or the human body). Moreover, the abovementioned windowsmay overlap one another over around 20% of their length along the Xaxis.

In the course of this cross-correlation process, a cross-correlationfunction <Sj(x,y), Sj+1(x,y)> is maximized so as to determine thedisplacement undergone by each particle 5 giving rise to an ultrasoundecho, in the direction X.

Examples of such cross-correlation calculations are given in the stateof the art, in particular by O'Donnell et al. (“Internal displacementand strain imaging using speckle tracking”, IEEE transactions onultrasonic, ferroelectrics, and frequency control, vol. 41, No. 3, May1994, p. 314-325) and by Ophir et al. (“Elastography: a quantitativemethod for imaging the elasticity of biological tissues”, Ultrasonicimag., vol. 13, p. 111-134, 1991).

A succession of displacement fields Djx(x,y) of the medium 1 under theeffect of the shear wave, in the direction X, is thus obtained.

This succession of displacement fields is stored in the memory M or inthe microcomputer 4 and can be viewed, in particular, by means of thescreen 4 a of the microcomputer, in the form of a slow-motion film wherethe value of the displacements is illustrated by an optical parametersuch as by a gray level or by a chromatic level.

The differences in propagation of the shear wave between the areas ofdifferent characteristics of the medium 1, for example healthy tissuesand cancerous tissues in the case of a medical application, are thusviewed perfectly.

This shear wave propagation film can moreover be superimposed with aconventional echography image, which may be generated by the devicedescribed above, which is able to operate:

either in the shear wave imaging mode,

or in a standard echography mode, as a function of the commands receivedfor example from the keyboard 4 b of the microcomputer.

Moreover, it is also possible to calculate, rather than thedisplacements of each point of the observed medium 1, the strainsEjx(x,y) of the medium in the direction X, that is to say thederivatives of the displacements Djx(x,y) with respect to x.

These successive strain fields are usable as before to clearly view thepropagation of the shear wave in the form of a film, and furthermoreexhibit the advantage of dispensing with the displacements of the probe6 with respect to the observed medium 1.

This variant is especially beneficial in the embodiment of FIG. 2, wherethe probe 6 is carried by the loudspeaker or vibrator 2, thisnecessarily involving motions of said probe since it is the latter whichitself then generates the shear wave.

On the basis of the displacement or strain fields, it is possible, ifappropriate, to proceed moreover to a mapping step in the course ofwhich, on the basis of the evolution of the motion parameter(displacement or strain) over time in the observation field X, Y (or Xin the case of a single transducer, or X, Y, Z in the case of a planearray of transducers), at least one shear wave propagation parameter iscalculated, either at certain points of the observation field which arechosen by the user from the microcomputer 4, or throughout theobservation field.

The shear wave propagation parameter which is calculated in the courseof the mapping step is chosen for example from the speed Cs of the shearwaves, the shear modulus μ, the attenuation α of the shear waves, theshear elasticity μ1, shear viscosity μ2.

This calculation is performed via a conventional inversion process, anexample of which is given hereinbelow in the case of a two-dimensionalobservation field (the same process would apply mutatis mutandis in thecase of an observation field having one or three dimensions,respectively for a single transducer T1 or for a plane array oftransducers).

In this example, the approximation will be made that the shear viscosityμ2 is zero and that the medium is isotropic.

The wave equation giving the displacement vector D of each point of themedium 1 may be written: $\begin{matrix}{{{\rho \quad \frac{\partial{\,^{2}\overset{\_}{D}}}{\partial t^{2}}} = {\Delta \quad \left( {\mu \cdot \overset{\_}{D}} \right)}},} & (I)\end{matrix}$

where ρ is the density of the medium 1, μ is the shear modulus (byassumption reduced to its real part the shear elasticity μ1 since theshear viscosity μ2 is assumed to be zero).

For the first component u of the vector D, that is to say for thedisplacement of the medium 1 in the direction X, we therefore have:$\begin{matrix}{{\rho \quad \frac{\partial{\,^{2}u}}{\partial t^{2}}} = {\frac{\partial{\,^{2}\left( {\mu \quad u} \right)}}{\partial x^{2}} + {\frac{\partial{\,^{2}\left( {\mu \quad u} \right)}}{\partial y^{2}}.}}} & ({II})\end{matrix}$

After temporal Fourier transform and discretization, this equation canbe written in the following matrix form, which can be written for eachfrequency of the spectrum of the shear wave:

{overscore (B)}=H.{overscore (M)}  (III), where:

M is a vector of dimension (L+2)·(M+2)−4, each component of which equalsμ_(lm), that is to say the local value of the shear modulus at eachdiscretized point with coordinates (x_(l),y_(m)), where l and m areintegers lying between 0 and respectively L+1 and M+1, eliminating thepairs (l,m) equal to (0,0), (0,M+1), (L+1,0) and (L+1,M+1), L+2 and M+2being the numbers of discretized points in the image of the medium 1respectively along the X and Y axes,

B is a vector of dimension L·M, whose components equal −{overscore(ω)}²·ρ·U_(lm) where {overscore (ω)} is the angular frequency of thelow-frequency shear wave, ρ is the density of the medium, U_(lm) is thetemporal Fourier transform of the displacement u at the point withcoordinates (x_(l),y_(m)), l lying between 1 and L and m lying between 1and M,

and H is a matrix of dimension L·M rows by (L+2)·(M+2)−4 columns, all ofwhose components are also known from the wave equation.

By juxtaposing a sufficient number of equations (III) correspondingrespectively to various available items of the frequency spectrum of theshear wave, a global matrix equation is obtained which can be solved bymatrix inversion, to obtain the vector M, that is to say the value ofthe shear modulus μ at every point of the observation field.

The local value of the speed of propagation Cs of the shear wave at eachpoint can then be deduced therefrom, if desired, through the formula:${Cs} = {\sqrt{\frac{\mu}{\rho}}.}$

The mode of calculation would be the same if using the strains of theobserved medium 1 rather than the displacements.

We claim:
 1. A method of imaging using shear waves to observe adiffusing viscoelastic medium which contains particles reflectingultrasonic compression waves, in which method an elastic shear wave isgenerated in the viscoelastic medium and the displacement of theviscoelastic medium subjected to said shear wave is observed by means ofat least one ultrasonic compression wave, wherein the shear wave isgenerated by applying to the viscoelastic medium an excitation havingthe form of a low-frequency pulse which exhibits a central frequency fof between 20 and 5000 Hz, this low-frequency pulse exhibiting aduration of between 1/2f and 20/f, wherein said method comprises apropagation observation step in the course of which the propagation ofthe shear wave is observed simultaneously at a multitude of points inthe observed medium, these points forming a substantially continuousobservation field extending at least along a first axis, this shear wavepropagation observation step consisting in: emitting into the observedmedium a succession of at least 10 shots of ultrasonic compression wavesat a rate of between 100 and 100 000 shots per second, detecting andrecording in real time the echoes generated by the reflecting particlesof the viscoelastic medium at each ultrasonic wave shot, these echoescorresponding to successive images of the observed medium, and whereinsaid method furthermore comprises a subsequent image processing step inthe course of which the images thus obtained are processed at a latertime at least by cross-correlation between successive images, so as todetermine at each point of the observation field a motion parameterchosen between the displacement and the strain of the viscoelasticmedium, in such a way as to thus obtain a succession of images showingthe evolution of the motion parameter of the viscoelastic medium underthe effect of the propagation of the shear wave.
 2. The method asclaimed in claim 1, in which the duration of the low-frequency pulse isbetween 1/2f and 2f.
 3. The method as claimed in claim 1, in which thecentral frequency of the low-frequency pulse is between 30 and 1000 Hz.4. The method as claimed in claim 1, in which the observed viscoelasticmedium consists of a living body comprising at least one internal organsubjected to pulsatile motions, the low-frequency pulse which generatesthe shear wave being constituted, by a pulsatile motion of said internalorgan.
 5. The method as claimed in claim 1, in which the observedviscoelastic medium is delimited by an outside surface and thelow-frequency pulse is applied in the vicinity of this outside surface.6. The method as claimed in claim 5, in which the low-frequency pulse isapplied by a means of excitation chosen from: an acoustic wave generatedby at least one acoustic transducer, and a shock generated locally byphysical contact in the vicinity of the outside surface of theviscoelastic medium.
 7. The method as claimed in claim 5, in which theultrasonic compression wave shots are emitted and the echoes generatedby the reflecting particles of the viscoelastic medium are detected bymeans of a bank of transducers which comprises at least one transducerand which is arranged in contact with the outside surface of theviscoelastic medium, the shear wave being applied to the viscoelasticmedium by imposing a pulsatile displacement on said bank of transducers.8. The method as claimed in claim 7, in which said motion parameter isthe strain of the viscoelastic medium.
 9. The method as claimed in claim1, in which, in the course of the observation of the propagation of theshear wave, between 100 and 10 000 ultrasonic compression wave shots areemitted at a rate of between 1000 and 100 000 shots per second.
 10. Themethod as claimed in claim 1, in which the observation field extends atleast along a plane comprising on the one hand, the first axis and onthe other hand, a second axis perpendicular to the first axis.
 11. Themethod as claimed in claim 10, in which, in the course of thepropagation observation step, a bank of several acoustic transducersarranged at least along the second axis is used to emit the ultrasonicacoustic compression wave shots and detect the echoes generated by thereflecting particles of the viscoelastic medium, the echoes detected byeach acoustic transducer being stored directly without prior processingin the course of the propagation observation step, and the imageprocessing step comprising a preliminary substep of forming paths in thecourse of which an image of the viscoelastic medium corresponding toeach ultrasonic compression wave shot is generated by combining at leastsome of the echoes received by the various transducers.
 12. The methodas claimed in claim 1, in which the image processing step is followed bya viewing step in the course of which a film consisting of thesuccession of processed images is viewed under slow motion, each pointof each image exhibiting an optical parameter which varies according tothe value of the motion parameter assigned to this point.
 13. The methodas claimed in claim 12, in which the optical parameter is chosen fromthe gray level and the chromatic level.
 14. The method as claimed inclaim 1, in which the image processing step is followed by a mappingstep in the course of which, on the basis of the evolution of the motionparameter over the course of time in the observation field, at least onepropagation parameter is calculated for the shear wave at at least somepoints of the observation field.
 15. The method as claimed in claim 14,in which the shear wave propagation parameter which is calculated in thecourse of the mapping step is chosen from the speed of the shear waves,the shear modulus, the attenuation of the shear waves, the shearelasticity, and the shear viscosity.
 16. An imaging device using shearwaves to observe a diffusing viscoelastic medium which containsparticles reflecting ultrasonic compression waves, this devicecomprising means of excitation for generating an elastic shear wave inthe viscoelastic medium and means of acquisition for observing, by meansof at least one ultrasonic compression wave, the displacement of theviscoelastic medium subjected to said shear wave, wherein the means ofexcitation are adapted to apply to the viscoelastic medium an excitationhaving the form of a low-frequency pulse which exhibits a centralfrequency f of between 20 and 5000 Hz, this low-frequency pulseexhibiting a duration of between 1/2f and 20/f, wherein the means ofacquisition are adapted to observe the propagation of the shear wavesimultaneously at a multitude of points in the observed medium, thesepoints forming a substantially continuous observation field extending atleast along a first axis, said means of acquisition being adapted for:emitting into the observed medium a succession of at least 10 shots ofultrasonic compression waves at a rate of between 100 and 100 000 shotsper second, detecting and recording in real time the echoes generated bythe reflecting particles of the viscoelastic medium at each ultrasonicwave shot, these echoes corresponding to successive images of theobserved medium, and wherein said device furthermore comprises imageprocessing means adapted to process at a later time the images obtainedby the observation means, at least by cross-correlation betweensuccessive images, so as to determine at each point of the observationfield a motion parameter chosen between the displacement and the strainof the viscoelastic medium, in such a way as to thus obtain a successionof images showing the evolution of the motion parameter of theviscoelastic medium under the effect of the propagation of the shearwave.
 17. The device as claimed in claim 16, in which the means ofobservation comprise a bank of transducers which includes at least onetransducer and which is adapted to be arranged in contact with anoutside surface delimiting the viscoelastic medium, the means ofexcitation being adapted to impose a pulsatile displacement on said bankof transducers.
 18. The device as claimed in claim 17, in which saidmotion parameter is the strain of the viscoelastic medium.
 19. Thedevice as claimed in claim 1, in which the observation field extends atleast along a plane comprising on the one hand, the first axis and onthe other hand, a second axis perpendicular to the first axis, the bankof transducers comprising several transducers arranged at least alongthe second axis, control means being provided for selectively operatingthe device either in the mode of imaging by shear waves, or in astandard echography mode making it possible to acquire between 10 and100 images per second.